The purpose of this proposal is to better understand the mechanism underlying the process of multicellular assembly exhibited by cranial neural crest cells during facial morphogenesis. It is proposed that this process occurs by a self-assembly mechanism, and further, that the biochemical differences observed between cells after mosaic formation, arose as a consequence of the assembly process. To test this hypothesis the eukaryotic microorganism Dictyostelium discoideum was chosen for study. This organism forms a multicellular aggregate from genetically, biochemically and cytologically identical cells at a specific stage in its development. Since these cells migrate and aggregate in response to a chemotactic signalling system involving cAMP, the possibility exists that this system is the mechanism regulating cellular differentiation. To better understand this signalling system, experiments on the D. discoideum adenylate cyclase are described. Experiments are planned to explore the mechanism underlying the ATP induced loss of catalytic activity. One enzymatic reaction to be examined in some detail is the adenyl transfer activity. The product of the reaction is to be isolated, and characterized. Further, the role of the 5'-nucleotide phosphodiesterase in the deadenylylation activity will be studied using 4-nitrophenyl phenylphosphate. The effect of this phosphonate ester on adenylylation product formation, AMP formation and finally on adenylate cyclase activity will also be studied. Studies on the 4-nitrophenyl phenylphosphate activity itself will also be undertaken.